1. Field of the Invention
An aspect of the present invention relates to a manufacturing method of an active matrix organic light emitting diode (AMOLED) display and an apparatus for manufacturing the active matrix organic light emitting diode (AMOLED) display. More particularly, an aspect of the present invention relates to a method for fabricating an organic light emitting diode (OLED) display having improved surface flatness and thickness uniformity as well as an improved image quality at edge regions of a pattern.
2. Description of the Related Art
An OLED display is a display device that electrically excites fluorescent organic material for emitting light and forms an image by controlling a voltage or current of N×M numbers of organic light emitting cells.
Each of the light emitting cells include an anode electrode which is a hole injection electrode, an organic layer having an light emitting layer (EML), and a cathode electrode which is an electron injection electrode. Excitons are formed by combining holes and electrons implanted into an organic layer from the respective electrode, and an image is displayed when the excitons are reduced from an excited state to a ground state.
Generally, the organic layer is formed by a multi-layer structure including an electron transport layer (ETL), a light emitting layer and hole transport layer (HTL), and the multi-layer structure may further includes an electron injection layer (EIL) and a hole injection layer (HIL).
In the OLED display having the above described organic light emitting cells, the organic layer is designed to create three colors (i.e., red (R), green (G), and blue (B)).
In addition, the organic layer is generally formed by a vacuum evaporative deposition process using a shadow mask or by a conventional optical etching process.
However, the vacuum evaporative deposition process has limitations reducing the physical gap between the patterns, and it is difficult to form a minute pattern to a level of tens of μm's which is required to prevent the possible deformation of the mask.
When the optical etching process is applied, although it is possible to form the minute pattern, practical application becomes difficult since the property of the light emitting material forming the light emitting layer may be deteriorated by the developing solution or the etching solution.
Therefore, a thermal transferring method has been recently proposed to form the light emitting layer.
The thermal transferring method converts light emitted from a light source into thermal energy by which an image formation material is transferred to a substrate to form a color pattern. Therefore, to perform the thermal transferring method, a light source, a donor film and a substrate are required.
Relating to the thermal transferring method, U.S. Pat. No. 5,521,035 discloses a method for manufacturing a color filter for a liquid crystal display by a laser thermal transferring method.
In this patent, the color filter is manufactured by a laser induction thermal transferring method for transferring a color material from a donor film to a substrate such as a glass or a polymeric film. As a laser unit, an Nd:YAG laser system is used for transferring the color material to the surface of the substrate.
As shown in FIG. 1, the Nd:YAG laser forms a Gaussian beam B1 having an energy distribution of a Gaussian function. When a diameter of the Gaussian beam B1 is set large (approximately, above 60 μm), the inclination of the energy distribution is smoothly reduced as it goes away from the center point O.
Therefore, as shown in FIG. 2, when the light emitting layer is formed by imaging the Gaussian beam B1 having a predetermined diameter to a donor film 100 in an X-direction, the image quality at the light emitting layer corresponding to an edge region 110 of the donor film 100 is deteriorated compared to a central portion along a Y-direction.
When the energy of the laser beam is intensified to improve the image quality at the edges in order to solve the above problem, although the image quality at the edges may be enhanced, the surface of the image pattern becomes irregular since the energy is excessively increased at the central portion.
In addition, when the laser thermal transferring method is used, the light emitting layer needs to be more carefully formed compared to the color filter.
That is, in a case of the color filter, color materials being transferred to a substrate using the laser thermal transferring method are formed by distributing pigments for color change into a binder polymer (e.g., acrylic resin or epoxy resin), and at this time, a rate of concentration of the pigments is from 20 to 40%.
However, the binder polymer is simply pervious to light. Accordingly, types of binder polymer vary in order to form an appropriate color pattern, and color materials for forming an appropriate pattern may be formed by changing a molecular weight or a glass transition temperature (Tg) value. In general, the color materials have a 60 to 120° Tg value and a 1,500 to 5,000 molecular weight, and a color layer of the color filter formed by the color materials has approximately 1 to 2 μm thickness.
However, slight changes to the properties of the light-emitting material (e.g., Tg value and molecular weight) highly affect the quality of the OLED display.
Accordingly, it is preferable, but not necessary, to adjust the pattern quality by adjusting the laser transfer characteristics rather than by modifying the properties of the light-emitting material since a limit of pattern quality is controlled by the laser transferring conditions.
In addition, since the light emitting materials used in the OLED display have a molecular weight of approximately 10,000 to 100,000 and a Tg value of over 100°, a process for forming the organic layer using the light emitting materials is more difficult than a process for forming the color layer by using the color materials.
A desired thickness of the organic layer formed by the laser thermal transferring method is about 50 to 100 nm, which is thinner than the thickness of the color filter.
Accordingly, when the formed organic layer is thinner than the color layer, the laser beam transferring condition and energy distribution are more carefully controlled compared to when the formed organic is thicker than the color filter.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form part of the prior art already known in this country to a person of ordinary skill in the art.